The present invention relates to digital communication systems and more particularly to systems and methods for simplifying the implementation of Orthogonal Frequency Division Multiplexing (OFDM) transmitters and receivers. The present invention also generally relates to computation of transforms such as the Fast Fourier Transform (FFT) and the Inverse Fast Fourier Transform (IFFT).
Wireless communications represent a highly useful alternative and supplement to wired services. Wireless offers access to untethered devices and thus addresses the reality that the modern worker often does not operately solely at a fixed workstation but rather moves frequently from location to location throughout the day. The remote location may be a conference room near the employee's desk, a client location across town, an airport, or a hotel room in another city or country.
Furthermore, for both office and home environments, even where computing devices have fixed locations, it may nonetheless be difficult or expensive to extend network access with the desired bandwidth everywhere as desired. The owners of older homes do not wish to incur the expense of rewriting for modern network technologies. As business grow and evolve, the demand for network connections may not correspond well to previously established wiring patterns.
Wireless communication is also highly useful in providing broadband Internet access to business and residences. A significant fraction of business and residences cannot get broadband Internet access through either Digital Subscriber Line (DSL) technology or cable modems. Wireless communications allows for broadband Internet access to be provided to these locations without the need to construct a new wired infrastructure.
As data rates increase to accommodate enhanced data communication services, multipath transmission effects becomes an obstacle to effective use of radio techniques for wireless communications. Within the most useful portions of the spectrum, a wireless signal will take two or more paths between the transmitter and the receiver due to reflections caused by terrain, foliage, buildings, indoor walls, etc. Because the paths are not the same length, there will be a difference in arrival times and the received wireless signal will interfere with time-shifted versions of itself. A further complication then arises because at particularly high data rates the self-interference will be between different data symbols transmitted at different times.
Orthogonal frequency division multiplexing (OFDM) is a highly powerful modulation technique that addresses the problems posed by multipath interference. To accomplish this, OFDM divides the available spectrum into many low data rate, narrow sub-channels that are transmitted in parallel. Since each sub-channel has a much lower data rate compared to the overall data rate the immunity to multipath is improved. At the same time interference between each sub-channel is eliminated by ensuring that the carrier frequency of each sub-channel is orthogonal to every other sub-channel. This is achieved by selecting sub-carriers to be a “frequency bin” used within an Inverse Fast Fourier Transform (IFFT). Incoming data is mapped to a constellation point for each sub-carrier or “frequency bin”. The IFFT is then used to convert the frequency domain OFDM symbol into N time domain samples where N is the size of the IFFT that was used.
A guard interval or cyclic prefix is added to complete an OFDM symbol (or block). The cyclic prefix is used to prevent inter-symbol interference and maintain the orthogonality between each OFDM symbol. The cyclic prefix is usually selected to be at least as long as the duration of the impulse response of the channel. It is created by copying the last L-samples of the time domain signal and appending this to the beginning of the OFDM block. A set of subchannel values are input to the IFFT procedure within the transmitter and recovered by use of an FFT at the receiver. These subchannel values are complex values that are chosen from a constellation of points to communicate data. The more points there are in the constellation, the more bits of data that may be sent in each sub-channel per OFDM symbol time.
The complex samples that are output by the IFFT are used to define the amplitude and phase of the real sinusoidal, composite signal carried over the airwaves. In the simplest approach, the real and imaginary components of the IFFT output samples are separately converted from digital form to analog form. For each time domain sample, the real value of the symbol defines the amplitude of an in-phase “I” component of the modulated signal while the imaginary value defines the amplitude of a quadrature “Q” component that is 90 degrees phase shifted from the “I” component. For high data rate systems, a relatively dense constellation is used on each sub-carrier and it is absolutely critical that the I and Q components are scaled identically at each stage of processing within both the transmitter and the receiver. Any imbalance between the I and Q channels will lead to distortion and performance degradation.
Because it is difficult to achieve perfect balance between the I and Q channels using analog circuits an alternative approach has emerged. In this approach, the digital baseband samples are interpolated to a higher sampling frequency, filtered to remove images and then upconverted to an Intermediate Frequency (IF). All this is done in the digital domain, not the analog domain, and thereby eliminates the variations associated with analog circuits. The digital signal is then converted to a passband analog signal using a Digital Analog Converter (DAC). This results in an oversampled analog signal centered at the selected IF. Since the result of this digital up conversion is a real passband signal, this process obviates the need for precisely balanced analog processing of separate I and Q channels and eliminates the need for two DACs. A similar approach is taken at the receiver where a passband signal is digitized at a low IF using an Analog to Digital Converter (ADC). This signal is then down converted to baseband and processed by a digital decimation filter to remove adjacent channel signals and reduce the sampling rate.
Problems arise, however, in the digital implementation of this prior art approach. In order to sufficiently separate sampling images and adjacent channel signals from the desired signal, long and computational intensive digital filters are required. This leads to higher power consumption, increased gate count, and more expensive demodulation hardware for the system as a whole.
What is needed are systems and methods for improving OFDM implementation in the areas of, e.g., power consumption, communications performance, and computational complexity.